The present invention relates to high voltage power supplies used in X-ray imaging equipment, and more particularly to circuitry for controlling an inverter used in such power supplies.
During an X-ray exposure, the high voltage across the anode and cathode of the X-ray tube must be carefully regulated. Such regulation not only is required to insure that a proper X-ray exposure occurs, but also to insure that an excessively high and harmful dose of X-rays is not produced. Typically, the regulation is performed by sensing the output of the high voltage supply for the X-ray tube. The sensed voltage is compared to the desired voltage for the selected exposure parameters. The result of that comparison is used to control the production of the high anode-cathode voltage that excites the X-ray tube.
The high voltage power supply can utilize a series resonant inverter circuit, a common type of which is referred to as an "H bridge." The load for the inverter is connected in the horizontal branch of the H bridge in series with inductance and capacitance, and each of the four vertical branches of the H has an electrically operated switch. A high DC voltage is applied across the branch ends at the top and bottom of the H. An alternating voltage can be applied across the load, by rapidly alternating the state of the diagonally opposed pairs of electrical switches. The alternating voltage produced by the inverter is coupled to a voltage multiplier, which increases the voltage to a level necessary for proper excitation of the X-ray tube and production of X-rays.
The inverter is operated by a control circuit, which receives a command reference signal indicating the level of high voltage required for the exposure selected by the operator. In addition, the control circuit receives a measurement of the output voltage produced by the voltage multiplier. This information is used by the control circuit to produce a particular drive frequency for the switch elements of the inverter.
A common high voltage supply for an X-ray tube utilizes a separate inverter and voltage multiplier for the cathode and the anode bias potentials with the output of the two voltage multipliers connected in series with a ground node in between. The anode inverter often is adjusted so that the anode voltage multiplier produces a higher output voltage than the cathode inverter and voltage multiplier combination for a given inverter drive frequency. In this arrangement, the output of the anode voltage multiplier is compared to the command reference signal to derive an error signal that forces adjustment of the anode inverter's duty cycle to reduce the output voltage from the anode voltage multiplier to achieve the desired voltage level for exciting the X-ray tube. However, this method presents a problem during recovery from a high voltage breakdown of the X-ray tube, commonly referred to as a "spit." The response time is necessarily quite slow since it is governed by the band pass feedback loop stability constraints. The slower the recovery time results in loss of X-ray data or in the case of a computed tomography system, loss of views.
This control method also requires careful adjustment of the inverter resonant frequency for both the anode and the cathode. A loss of control can result if the adjustment is made incorrectly.